Our long-term objective is to understand how animals recognize chemical cues present in the environment or released by mates, competitors and predators and how these cues mediate innate behaviors. Chemosensory perception provides all organisms, from bacteria to humans, with essential information about the chemical composition of the external world. In most animals, this 'chemical world' is perceived by two distinct sensory modalities: the sense of taste (gustation) and the sense of smell (olfaction). Insects show a wide range of feeding preferences that can be extremely narrow and be restricted on a single host plant, or fairly broad and include many plants and even animal cadavers. Some insects, including Drosophila, have remarkably similar taste preferences as mammals. Our general strategy is to utilize the genetically amenable system of Drosophila melanogaster to study the perception of taste. Drosophila gustatory neurons express divergent members of G-protein coupled receptors (GPCRs) that interact with non-volatile, soluble ligands. Activation of these cells is propagated to specific taste centers in the brain, where the received information is translated into a behavioral output (feeding/sucking, avoidance, courtship). Behavioral investigations and electrophysiological studies have indicated that Drosophila has a well-developed taste sensory system that can recognizes a large number of different substrates (ligands) using a large family of about 70 gustatory receptor (Gr) genes encoding divergent GPCRs. Expression studies of a small number of Gr genes revealed that a given gene is expressed in a distinct small set (1- 4 %) of gustatory neurons, creating a peripheral map in the insect gustatory system. Thus, cellular segregation of individual taste receptor genes provides a concept for high discriminatory power of different substrates that are preferentially recognized by cognate receptors. Hence, these observations have, led to the hypothesis that Drosophila and other insects recognize and distinguish significantly more taste qualities than mammals. To test this hypothesis, we propose a systematic analysis of the Drosophila taste receptor gene family by determining the global Gr expression profile and their ligand specificity. Moreover, we propose to test whether the expression map in the peripheral sensory system is translated into a functional map in the subesophageal ganglion. This research proposal has been modified based on the reviewer's recommendations, recently published reports and new data generated in our laboratory.